1. Field of Invention
The present invention relates to a method for using a flash to assist in focal length detection, and more particularly to a method for looking up an exposure feature value table to assist in detecting a focal length of an image having a face.
2. Related Art
When a camera is used to take an image, the camera needs to correctly focus on an object to be photographed so as to obtain a clear picture. Nowadays, common digital cameras have an auto focus function. The auto focus may substantially be divided into two types: one is active auto focus, and the other is passive auto focus.
The technique of the active auto focus is to use a set of infrared transmitter or laser transmitter and corresponding receiver to project a pattern of light on an object to be photographed, and then calculate a distance between the camera and the object to be photographed through a method such as triangulation. This distance is a focal length. Since the optical transmitter and receiver need to be additionally set in the camera when the active auto focus is used, the problem of high cost occurs.
In the passive auto focus, before the actual photographing, a camera lens of the digital camera is moved to a plurality of focus positions corresponding to the range from a minimum focus distance closest to the digital camera to infinity. The digital camera captures an image at the focus position and analyzes a definition of the acquired image to determine a focal length. The passive auto focus only needs to use an image sensor and an operation unit of the camera, and thus the manufacturing cost can be reduced.
However, since the passive auto focus has to rely on the definition calculated from the pre-captured image, when the object to be photographed is in an environment with insufficient brightness, the estimated focal length may have an error easily. In the photographing, sufficient light enables the object to be photographed to have an obvious in-focus position, such that the camera easily finds the correct focus position. On the contrary, if the environment has insufficient brightness, the camera cannot find the correct focal length easily because the definitions of the images are generally low when the definitions are estimated. Thus, the passive auto focus has the problem of less accurate focusing when the environment is not bright enough.
Besides, when a picture is taken, a human image is a relatively important part in the picture, so operations such as focusing or light metering should be performed based on a human face before image photographing. However, the conventional auto focus methods may take the background or things other than the human as the object to be focused, such that the human which should be the key for photographing is out of focus.